This invention relates to a sensor element for detecting trace amounts of smoke and/or gaseous reducing substances contained in the atmosphere, exhaust gases, etc.
For detecting these trace amounts of smoke and/or reducing gases, there are such conventionally known methods as gas chromatography and a method which makes use of a semiconductor element. Gas chromatography, however, cannot be said to be a convenient and inexpensive detecting method because it requires a large-scale apparatus and a certain degree of skill in the analytical procedure.
Among the detecting methods which make use of a semiconductor element for the sensor, there is known, for example, a method which utilizes the change in the specific resistance of a shaped piece comprising stannic oxide as the main constituent. This method, however, has the disadvantages that the sensor has an extremely large temperature coefficient of resistance (from 2.5 .times. 10.sup.6 .OMEGA. at 25.degree. C. to 10.sup.4 .OMEGA. at 250.degree. C. in air), and, in addition, the resistance of the sensor changes with changing water vapor content.